In recent years, in a portable terminal device on which a touch panel is mounted, a predetermined operation is performed generally by touching the touch panel arranged on the display surface of the portable terminal device. In such a portable terminal device, more various and convenient operation procedures are demanded. Besides, many users dislike having the display surface of the portable terminal device hidden behind fingers, or dislike having the display surface soiled by fingerprints. Therefore, a portable terminal device that has a function of performing a predetermined operation when a user touches a side surface or a back surface of the portable terminal device is desired.
Further, in order to cause a predetermined operation to be performed according to a user's hand holding the portable terminal device so that the operability is to be improved, or in order to prevent a malfunction that is caused by a user unintentionally touching the display surface of the portable terminal device, it is useful to develop techniques for providing sensors on a side surface or a back surface of the portable terminal device so as to cause the sensors to recognize the state of the user's hand holding the portable terminal device, so that the operability is improved or a malfunction prevention function is realized. For this purpose, techniques such as those disclosed in Patent Document 1 (JP-A-11(1999)-143604), Patent Document 2 (JP-A-2010-154090), and, Patent Document 3 (WO2012/049942) have been developed.